Primitives: fixed cleanup by setting "alive" flag after Body creation

I was getting strange behaviour and crashes on exit, because primitives (the three types derived from `ofxBox2dBaseShape`) that had been deleted in my application were still being referenced by the World. The `alive` flag for Circle, Rect and Polygon was never set, so the call to `destroy()` was always being skipped in the destructor.

It seems it's been this way for a long time.  A more complete fix might be to remove one of the two flags (`dead` and `alive`) -- seems strange to have one of each, and all the more since they weren't really used for anything.